For certain applications, variable gain amplifier circuits need to provide fine gain variation, i.e. fine variation of the ratio of resistors within the respective circuits.
Several attempts have been made to vary the gain in such variable gain amplifier circuits. FIG. 1 is a schematic diagram of a prior art circuit 10 and represents one such prior attempt. The circuit 10 includes a string of resistors, of which resistors R1 through R7 are shown, and an operational amplifier 11. The negative input of the amplifier 11 is coupled to multiple switches, of which switches S1 through S6 are shown, each switch being also connected into the string of resistors R1 through R7. The output of the operational amplifier 11 is coupled to the feedback resistor R7. In FIG. 1, the value of the gain depends on the particular switch being closed. For example, if switch S1 is closed, the gain can be calculated with the formulag1=R2+R3+ . . . +R7/R1 Similarly, if switch S2 is closed, the gain can be calculated as g2=R3+ . . . +R7/R1+R2.
Thus, fine variation of the value of the gain can be achieved if large numbers of resistors and switches are provided in the circuit 10, which leads ultimately to very complex and expensive circuits.